Scanners and copiers use document feeders to transport documents into the machine. Mechanisms used for the transportation of documents, including paper or sheets of other material, have the capacity to accidentally pick up more than one document fed from a stack of documents. It is necessary to determine when more than one document is pulled into a document transport since multiple documents may jam the transport or prevent processing some documents. In many cases, the documents fed into the scanner are different sizes. Some smaller documents may be located on one side of a document feed path and not pass under a single position sensor.
There are two general methods for multiple document detection, contact and non-contact. The contact methods include measurement of small thickness changes with a contact foot or sensing arm that is in contact with the documents as they pass through the document transport. The contact foot is connected to a linear voltage differential transducer (LVDT), or a magnet, which is sensed by a Hall Effect Sensor. These sensors can detect changes in thickness of less than 1 μm (10−6 m).
The major disadvantage to the contact method is that anything in contact with moving paper, especially thin paper or ripped paper, can cause a malfunction such as a paper jam. The contact method also requires calibration using the maximum thickness document that will be fed through the document transport. When a thickness is measured which is above the calibration value plus a threshold, typically 30%, it is determined to be a multiple document feed. This method, however, will only work when documents having a uniform thickness are processed. Using a wheel on the end of the contact foot can reduce the chances of paper jam, however, the variations in the diameter of this wheel, due to the nonconformity in manufacturing, must be taken into account during the measurements.
The primary non-contact method for multiple document detection sends ultrasound signals through the document stream to determine if more than one document is present. Sending ultrasound through paper results in attenuation of the ultrasound signal. It is possible to determine the presence of multiple documents by change in attenuation of the signal received. This method is independent of the thickness of the individual documents and is made without making contact with these documents.
There are currently ultrasonic detection system available, which use high frequency sensors to sense multiple zones within a local area. This approach works because the sensors are directional, and the signal from one sensor does not interfere with the signal from other sensors. However, these sensors are also more expensive. Low cost sensors have a wide angle of energy emitted, and if used, the sensors can interfere with each other (cross-talk). The interference often causes the design to fail. The problem cannot be solved by multiplexing individual pulses to the emitters, because not enough energy will be sent by the detector and the time phasing of the signal is not stable.
U.S. Pat. No. 6,212,130 uses a tilted ultrasonic sensor. This could be used in multiple locations but the cost would be significant, using the more expensive directional high frequency sensors.